Method of depositing an emissive coating on cathode structures



-ing scanning spot will be non-circular.

United States Patent- METHOD OF DEPOSETING AN ENIISSIVE COATING N CATHGDE STRUCTURES 'Vytautas'Andriulis and Anacleto D..Giacchetti, Chicago, llL, assignors 'to National Video Corporation, Chicago, Ill.

Application June 5, 1953, Serial No. 359,732

4 Claims. -(Cl. 117-212) Our invention relates to a new' and improved method of depositing the emissive coating ona. cathode support to-provide a cathodic emitter, for example, an indirectly heated cathode as used in a thermionic device such as a cathode-ray tube.

In the present commercial production of. a cup-shaped cathode for the electron gun structure of a television picture tube for example, the emissive coating is applied to the closed end of the cup by spraying and results in a cathode-having several objectionable features, viz: 1) the surface so produced is non-uniform, having a pocked or orange-peel character. Consequently, emission occurs predominantly from the high points of the surface (under ordinary conditions of operation) and, since such high points are inevitably non-uniformly distributed the result- As a result, picture quality, i. e. definition, suffers; (2) the'thickness of a sprayed layer cannot be controlled with any degree of precision, and a further undesirable condition is-encountered because the cutoff voltage depends to agreat extent on the axial spacing between the surface of the cathode and the adjacent wall of the control grid stop. Hence, each cathode must be measured and paired with the proper sized grid-cathode spacer to insure maintaining the allowable tolerance on the cutofi voltage.

These serious disadvantages of the present commercial procedure for producing cathodes are substantially eliminated by our invention.

According to our invention, the emissive material is applied to the cathode support'by gravity settling thereof out a liquid suspension. By this method the useful surface of the layer can be held substantially fiat to within a few microns. Furthermore, the thickness of the coating may be controlled to substantially any desired value.

Briefly outlined, in accordance with our invention, a plurality of the cathode supports are inserted into openings provided in the floor of a container, which openings are of such size as snugly to receive the supports, e. g. cylindrical cups the closed end whereof are allowed to project a slight distance into the container and to seat snugly in the openings so that there will be no leakage of consequence. At least that portion of the container which receives the supports is selected to provide the desired characteristics of yieldability, and chemical inertness to the liquid therein contained. A liquid suspension of the coating material is poured into the container and permitted to settle out, depositing on the ends of the supports purely by gravity. Excess liquid is then decanted or drained and the coating is hardened.

Our invention will best be understood by reference to the appended drawings forming a part of this specification, illustrating only one preferred means for carrying out our invention, and wherein Fig. 1 is a sectional view in perspective of a tank having cathode supports arranged therein, in process of being coated in accordance with our invention;

Fig. 2 is a fragmentary perspective view, on an enlarged scale, showing a single support as it is arranged in 2,739,911 Patented Mar. 27, 1956 the tank and after the coating has been applied thereto in accordance withour invention;

Fig. 3 is a fragmentary sectional view, on an enlarged scale, of the coated cathode and tank; and I Figs. 4 and 5 show alternativeconstructions of the floor of the tank.

For the practice of our invention we provide a tank or container 10 which is formed of a suitablematerial having the qualities of resiliency together with a certain degree of rigidity, and for reasons which will best be understood asthe description proceeds. We preferably employ a synthetic plastic material, such as a polyethylene ester, but other substances such as natural or synthetic rubber or the like may likewise be used. The container is preferably parallelepipedic, having side walls12 and a floor 13, the latter being perforated'as at 15 "to provide a plurality of bores of"substantially the same diameter as the cathodes 18 which are to be coated, or slightly smaller. In the event the portion of the cathode support retained in the bore is of other than circular cross section it will be-appreciated that the perforations will be correspondingly modified. A suitable cock (not shown) may *are pressed into-the bores 15 to the extent that the closed ends 19 will project a slight distance, say about 0.5 mm., above the floor 13 of the tank. Inasmuch as the cathode supports and the holes are of substantially the same diameter, the former willbe snugly received in the bores substantially to seal the same against leakage of liquid without the'necessity for gaskets or the like, it-being evident that seepage is prevented by reason of the selection of a'material for the tank such that its inherent resiliency will 'allowthe same to yield while the supports are forced into place.

After the supports have been arranged inthetank, a "liquid suspension of the coating material is poured thereinto. The emissive material may be of the character now employed forcoating cathodes, or any other suitable material. Usually the formula comprises co-precipit-ated barium carbonate and strontium carbonate, as the active soluble, but which solveritwill be physically and chemically inert to the emitting-material. The preferred liquid medium is acetone, but obviously other organic liquids having the aforesaid properties may likewise be employed.

A product having the desired characteristics is available from J. P. Baker Chemical Co., of Philipsburg, N. J., under the designation R 500, and having amyl acetate and other unknown organic solvents as its liquid medium. For our purposes, we decant the liquid medium and add acetone and nitrocellulose binder.

The particles of emissive material will settle out of the suspension in a relatively short time, usually a few minutes, and the excess liquid is then partially or entirely removed either by decanting or draining the same out of the cock. A small amount of the liquid may be permitted to remain in the tank without adverse effect.

The coating which has deposited on the cathode supports by gravity settling is then hardened by pouring into the tank a water-acetone mixture containing, say, about 3.5 parts water to about 10 parts acetone by volume; the total amount of such solution is not critical, it being necessary merely to cover the supports by an inch or so of liquid. The hardening operation takes place by the water eliminating the acetone from the deposited material, whereupon the coating becomes quite firm. The hardening liquid is allowed to remain in the container in contact with the cathodes for about 5 minutes and is, then removed.

The cathodes are then removed from their respective bores by withdrawing the same from the bottom, are permitted to dry in the air and are then ready for use.

One of the important considerations involved in practicing the invention is the confinement of the coating to the end face of the cathode, for otherwise material lodging on the curved surface of the cathode can detract from the desired circular beam by providing fringe zones of electron emission. Although the stop confronting the cathode is provided with an aperture of predetermined size to shape the beam, impingement upon the margin of the aperture of excess electrons, i. e. electron current, may, under some conditions, burn the aperture.

The foregoing desideratum is attained by assuring that the floor of the tank, and hence, the end face of the cathodes, is horizontal. Thus, gravity is effective to deposit the material upon only the flat end face of the cathode. Although we show a tank having a flat floor it will be apparent that crateriform protuberances 21 or 22 may be used (Figs. 4 and 5). In any case it is preferred to position the flat face of the cathode above the surrounding surface of the tank to insure that none of the coating material may adhere to the circular boundary of the cathode and thereby provide an erose margin around the desired circular emissive are.

Some of the precautions necessary to be observed in connection with our improved process are elimination of dust from the vicinity of the settling material, such as by providing a cover therefor, avoidance of jarring of the settling tank, such as by supporting the same on a shockmount, avoidance of turbulence in the settling medium until the cathode material has hardened, and preservation of horizontal positioning of the end face of the cathode.

The entire operation may take place at normal room temperature.

From the above description it will be seen that, in accordance with our invention, we are able to produce vastly improved cathodes at a greatly reduced cost. The necessity for skilled labor is eliminated, as any unskilled person may, with a minimum of instruction, perform the necessary operations. Furthermore, the apparatus required is of the simplest, most inexpensive character.

Where, in the description and claims we employ the r term cathode support," the same is intended to refer to the metallic element to which the emissive coating is applied. The terms cathode or cathodic emitter designate the completed cathode.

While we have shown particular embodiments of our invention, it will be understood, of course, that we do not wish to be limited thereto since many modifications may be made, and we therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of our invention.

We claim:

1. The method of depositing an emissive cathodic coating upon the fiat end surface of a cup shaped cathode support which comprises supporting the cathode support in a confined zone larger in diameter than the cathode support with the end surface of the cathode support uppermost and horizontal and above the bottom of the zone, introducing into the zone a suspension containing a particulate cathodic material and binder, maintaining quiescent conditions during settling of the material on said end, removing the excess suspension and hardening the coating while the cathode is thus supported.

2. The method of depositing an emissive cathodic coating upon the flat end surface of a cup shaped cathode support which comprises supporting the cathode support in a confined zone larger in diameter than the cathode support with the end surface of the cathode support uppermost and horizontal and above the bottom of the zone, introducing into the zone a suspension containing a particulate cathodic material and binder, maintaining quiescent conditions during the settling of the material on said end, and removing the excess suspension.

3. The method of depositing an emissive cathodic c0ating upon the flat end surface of a cup shaped cathode support which comprises supporting the cathode support in a confined zone bounded by a floor larger in diameter than the cathode support with the end surface of the cathode support uppermost and horizontal and above the floor, introducing into the zone a suspension containing a particulate cathodic material and binder, maintaining quiescent conditions during settling of the material on said end, removing the excess suspension and hardening the coating while the cathode is thus supported.

4. The method of depositing an emissive cathodic coating upon the flat end surface of a cup shaped cathode support which comprises supporting the cathode support in a confined zone bounded by a floor larger in diameter than the cathode support with the end surface of the cathode support uppermost and horizontal and above the floor, introducing into the zone a suspension containing a particular cathodic material and binder, maintaining quiescent conditions during settling of the material on said end, and removing the excess suspension.

References Cited in the file of this patent UNITED STATES PATENTS 293,650 Gorman Feb. 19, 1884 1,747,704 Glaudel Feb. 18, 1930 2,321,149 Koch June 8, 1943 2,615,821 Levy Oct. 28, 1952 

1. THE METHOD OF DEPOSITING AN EMISSIVE CATHODIC COATING UPON THE FLAT END SURFACE OF A CUP SHAPED CATHODE SUPPORT WHICH COMPRISES SUPPORTING THE CATHODE SUPPORT IN A CONFINED ZONE LARGER IN DIAMETER THAN THE CATHODE SUPPORT WITH THE END SURFACE OF THE CATHODE SUPPORT UPPERMOST AND HORIZONTAL AND ABOVE THE BOTTOM OF THE ZONE, INTRODUCING INTO THE ZONE A SUSPENSION CONTAINING A PARTICULATE CATHODIC MATERIAL AND BINDER, MAINTAINING QUIESCENT CONDITIONS DURING SETTLING OF THE MATERIAL ON SAID END, REMOVING THE EXCESS SUSPENSION AND HARDENING THE COATING WHILE THE CATHODE IS THUS SUPPORTED. 